Electrowetting on dielectric (EWOD) is a well-known technique for manipulating droplets of fluid by the application of an electric field. Active Matrix EWOD (AM-EWOD) refers to implementation of EWOD in an active matrix array incorporating transistors, for example by using thin film transistors (TFTs). It is thus a candidate technology for digital microfluidics for lab-on-a-chip technology. An introduction to the basic principles of the technology can be found in “Digital microfluidics: is a true lab-on-a-chip possible?”, R. B. Fair, Microfluid Nanofluid (2007) 3:245-281).
FIG. 1 shows a part of a conventional EWOD device in cross section. The device includes a lower substrate 10, the uppermost layer of which is formed from a conductive material which is patterned so that a plurality of array element electrodes 12 (e.g., 12A and 12B in FIG. 1) are realized. The electrode of a given array element may be termed the element electrode 12. A liquid droplet 14, including a polar material (which is commonly also aqueous and/or ionic), is constrained in a plane between the lower substrate 10 and a top substrate 16. A suitable gap between the two substrates may be realized by means of a spacer 18, and a non-polar surround fluid 20 (e.g. oil) may be used to occupy the volume not occupied by the liquid droplet 14. An insulator layer 22 disposed upon the lower substrate 10 separates the conductive element electrodes 12A, 12B from a first hydrophobic coating 24 upon which the liquid droplet 14 sits with a contact angle 26 represented by θ. The hydrophobic coating is formed from a hydrophobic material (commonly, but not necessarily, a fluoropolymer).
On the top substrate 16 is a second hydrophobic coating 28 with which the liquid droplet 14 may come into contact. Interposed between the top substrate 16 and the second hydrophobic coating 28 is a reference electrode 30.
The contact angle θ is defined as shown in FIG. 1, and is determined by the balancing of the surface tension components between the solid-to liquid (γSL), the liquid-to non-polar surrounding fluid (γLG) and the solid to non-polar surrounding fluid (γSG) interfaces, and in the case where no voltages are applied satisfies Young's law, the equation being given by:
                              cos          ⁢                                          ⁢          θ                =                                            γ              SG                        -                          γ              SL                                            γ            LG                                              (                  equation          ⁢                                          ⁢          1                )            
In operation, voltages termed the EW drive voltages, (e.g. VT, V0 and V00 in FIG. 1) may be externally applied to different electrodes (e.g. reference electrode 30, element electrodes 12, 12A and 12B, respectively). The resulting electrical forces that are set up effectively control the hydrophobicity of the hydrophobic coating 24. By arranging for different EW drive voltages (e.g. V0 and V00) to be applied to different element electrodes (e.g. 12A and 12B), the liquid droplet 14 may be moved in the lateral plane between the two substrates 10 and 16.
Example configurations and operation of EWOD devices are described in the following. U.S. Pat. No. 6,911,132 (Pamula et al., issued Jun. 28, 2005) discloses a two dimensional EWOD array to control the position and movement of droplets in two dimensions. U.S. Pat. No. 6,565,727 (Shenderov, issued May 20, 2003) further discloses methods for other droplet operations including the splitting and merging of droplets, and the mixing together of droplets of different materials. U.S. Pat. No. 7,163,612 (Sterling et al., issued Jan. 16, 2007) describes how TFT based thin film electronics may be used to control the addressing of voltage pulses to an EWOD array by using circuit arrangements very similar to those employed in AM display technologies.
The approach of U.S. Pat. No. 7,163,612 may be termed “Active Matrix Electrowetting on Dielectric” (AM-EWOD). There are several advantages in using TFT based thin film electronics to control an EWOD array, namely:                Electronic driver circuits can be integrated onto the lower substrate 10.        TFT-based thin film electronics are well suited to the AM-EWOD application. They are cheap to produce so that relatively large substrate areas can be produced at relatively low cost.        TFTs fabricated in standard processes can be designed to operate at much higher voltages than transistors fabricated in standard CMOS processes. This is significant since many EWOD technologies require electro-wetting voltages in excess of 20V to be applied.        
To perform various droplet operations in an AM-EWOD device, it can be desirable to be able to sense a droplet property, such as droplet size or location on the array of elements. US application 2010/0194408 (Sturmer et al., published Aug. 5, 2010) describes a method, circuit and apparatus for detecting capacitance on a droplet actuator, inter alia, for determining the presence, partial presence or absence of a droplet at an electrode. U.S. Pat. No. 8,653,832 (Hadwen et al., issued Feb. 18, 2014) describes how an impedance (capacitance) sensing function can be incorporated into the array element circuit of each array element of an AM-EWOD device. The impedance sensor circuit may be used for determining the presence and size of liquid droplets present at each electrode in the array.
Upon adequate sensing, droplet operations may then be performed, such as for example holding a droplet position, moving a droplet across the device, splitting a droplet into multiple droplets, mixing different droplets, and others. These various operations may be performed by actuating a suitable pattern of elements on the AM-EWOD device. For example, U.S. Pat. No. 7,569,129 (Pamula et al., issued Aug. 4, 2009) describes the use of element actuation to hold a droplet isolated from other droplets. Sturmer referenced above describes the use of capacitance detection as real time feedback to determine whether a droplet operation has been successful. WO2008055256A3 (Kim et al., published May 8, 2008) describes the use of capacitance detection as real time feedback to control the volume of a droplet being dispensed or split from a reservoir. Other actuation methods for performing various droplet operations are known.